1. Field of the Invention
The present invention relates to a method of chemical decontamination for the components of a radioactive material handling facility, such as a nuclear power station, and a system for carrying out the method of removing metal oxides containing radioactive nuclides and adhering to the components of the radioactive material handling facilities from the surfaces of the contaminated components by chemical dissolution.
2. Description of the Related Art
Oxide films containing radioactive nuclides are deposited or formed on the surfaces of components of a nuclear power station in contact with fluids containing radioactive nuclides during operation and subject to radioactive contamination, such as pipes, pieces of equipment and structural members. Consequently, the dose rate around those component members increases and the radiation exposure of workers engaged in work for periodic inspection or dismantlement of a nuclear reactor for decommissioning.
In order to remove the oxide film, a decontaminating solution is supplied the oxide film or a metal forming a contaminated object so as to dissolve them, thereby the oxide film is dissolved in the solution or peeled off into the solution. Aforementioned chemical decontamination method, which dissolves or removes the oxide film chemically, has practically been applied to the decontamination of the components of some nuclear plants and has produced satisfactory results in reducing mediation exposure.
Various chemical decontamination methods intended for the decontamination of stainless steel components of atomic energy plants have been proposed. One of those chemical decontamination methods comprises, in combination, a step of dissolving chromium oxides contained in an oxide film through oxidation using an oxidizing agent, and a step of dissolving ferrous oxides, which are principal components of the oxide film, through reduction a reducing agent.
A chemical decontamination method disclosed in JP B No. Hei 3-10919 employs permanganic acid as an oxidizing agent, and dicarboxylic acid as a reducing agent. The chemical decontaminating method using permanganic acid which has a high oxidizing effect in a low concentration and dicarboxylic acid which can be decomposed into CO2 and H2O produces less secondary wastes as compared with hitherto known chemical decontamination methods and has practically been applied to decontamination work in nuclear power plants.
A decontamination method disclosed in JP A No. Sho 55-135800 uses, in combination, an ozone solution prepared by dissolving ozone in water as an oxidizing agent, and a decontaminating liquid containing an organic acid and a complexing agent. A decontamination method disclosed in JP A No. Hei 9-151798 prepares a foamed decontaminating liquid by blowing ozone gas into a solution containing a foaming agent, and feeds the foamed decontaminating liquid into a contaminated object for decontamination.
When decontaminating contaminated objects by the chemical decontamination method using permanganic acid and dicarboxylic acid as decontaminating agents, the decontaminating agents produce secondary wastes in recovering manganese ion from the permanganic acid solution by means of an ion-exchange resin.
As generally known, ozone is a highly oxidative gas, reacts with water and is decomposed to produce various oxidative active oxygen species. The decontamination method will be a very effective method producing the least amount of secondary wastes attributable to an oxidizing agent if the oxide film can be dissolved in an ozone solution prepared by efficiently dissolving ozone gas in water. However, the ozone concentration of ozone gas produced by a conventional silent discharge ozonizer is low (in general, lower than 1% by volume), and the ozone concentration of ozone solution prepared by supplying the ozone gas in an acid solution is several parts per million or less.
An oxidation process using an ozone solution having such a low ozone concentration, as compared with a conventional chemical decontamination method using permanganic acid, has an inferior oxide film removing ability. To make matters worse, ozone decomposes in water and the ozone concentration of the ozone solution decreases. When the temperature of the ozone solution is high, the half life of dissolved ozone is short and it is possible that ozone disappears in a few minutes. The higher the temperature of the decontaminating liquid for the chemical decontamination method, the higher is the rate of dissolution of the oxide film and the higher is the decontaminating effect. Therefore, the chemical decontamination method must be carried out at temperatures as high as possible to reduce overall time necessary for decontamination work.
Although it is possible to hold ozone gas in foams produced in the decontaminating liquid by a foaming agent thereby holding ozone in the decontaminating liquid for a long time, the foaming agent produces secondary wastes.
The known chemical decontamination method using oxidation and reduction is applied mainly to decontaminating stainless steel components and cannot be applied to decontaminating components made of metallic materials susceptible to the corrosion by chemicals, such as carbon steels.
The present invention has been made to solve the foregoing problems and it is therefore an object of the present invention to provide a chemical decontamination method and a system for carrying out the same capable of chemically decontaminating components of radioactive material handling facilities and of efficiently dissolving oxide films through oxidation, and producing only a small amount of secondary wastes.
Another object of the present invention is to provide a chemical decontamination method and a system for carrying out the same capable of decomposing organic acid used as a decontaminating agent, such as oxalic acid, and exhaust ozone gas.
To achieve the objects, according to a first aspect of the present invention, a method of removing an oxide film containing radioactive nuclides and adhering to a component of a radioactive material handling facility is provided. The method includes an oxidative dissolving process for dissolving the oxide film through oxidation using an ozone solution prepared by bringing ozone gas into contact with an acid solution.
Preferably, the ozone solution has a pH value of 6 or below, more preferably, 5 or below.
Oxide films deposited or formed on the surfaces of contaminated components, such as pipes and pieces of equipment of a radioactive material handling facility, can effectively dissolve and removed by using a solution prepared by dissolving ozone, i.e. an oxidative gas, in water of a desired quality.
Preferably, the working temperature of the ozone solution for the oxidative dissolving process is in the range of 50 to 90xc2x0 C.
Preferably, the ozone gas is produced by an electrolytic ozonizer that has an anode chamber formed on one side of a solid electrolyte and a cathode chamber formed on the other side of the solid electrolyte, and generates ozone in the anode chamber by a solid electrolyte electrolytic process in which pure water is subjected to electrolysis using an anode of a catalytic metal disposed in the anode chamber.
The method may further include a monitoring process for measuring the oxidation-reduction potential of the ozone solution to monitor the oxidative dissolving ability of the zone solution.
The method may further include a reductive dissolving process in which a decontaminating solution, such as an organic acid solution, is supplied to the contaminated object for the reductive dissolution of the oxide film. The amount of secondary wastes originating in decontaminating agents can be reduced by using ozone in the oxidative dissolving process and using an reductive organic acid capable of being decomposed into CO2 and H2O in the reductive dissolving process.
The method may further include a reducing agent decomposing process for decomposing an organic acid remaining in the organic acid solution after the reductive dissolving process, and an ion removing process for removing ions remaining in the ozone solution or in the organic acid solution.
The reducing agent decomposing process may include the steps of adding at least either ozone or hydrogen peroxide to the organic acid solution, and irradiating the organic acid solution with at least either ultraviolet rays or radioactive rays. The organic acid may be decomposed by using the photocatalytic action of titanium oxide in the reducing agent decomposing process by irradiating titanium oxide with light and bringing titanium oxide into contact with the organic acid solution instead of using those steps.
The method may further include an oxidizing agent decomposing process for decomposing ozone contained in the ozone solution by irradiating the ozone solution with ultraviolet rays or radiation after the oxidative dissolving process.
The organic acid solution used in the reductive dissolving process may contain a salt of the organic acid contained in the organic acid solution in addition to the organic acid. For example, the use of a solution containing oxalic acid and an oxalate enables the application of chemical decontamination to the decontamination of carbon steel members susceptible to corrosion.
According the second aspect of the present invention, a decontamination system, for removing an oxide film containing radioactive nuclides and adhering to a contaminated object, i.e., a component of a radioactive material handling facility, is provided. The system includes: a decontaminating liquid circulating system provided with a pump for circulating a decontaminating liquid through the contaminated object, an ozone supply system for supplying ozone to the decontaminating liquid circulating in the decontaminating liquid circulating system, a pH adjusting agent supply device for supplying a pH adjusting agent to the decontaminating liquid circulating in the decontaminating liquid circulating systems, an organic acid supplying device for supplying an organic acid as a reducing agent to the decontaminating liquid circulating in the decontaminating liquid circulating system, an irradiating device for irradiating the decontaminating liquid circulating in the decontaminating liquid circulating system with light, and an ion-exchange device for removing ions contained in the decontaminating liquid circulating in the decontaminating liquid circulating system.
According the third aspect of the present invention, a method of removing an oxide film containing radioactive nuclides and adhering to contaminated objects, the contaminated objects including a reactor coolant pump for circulating a coolant for cooling a nuclear reactor, and a pipe having sections connected to an inlet side and an outlet side of the coolant circulating pump, respectively, and rising to a level higher than that of the reactor coolant pump, is provided. The method includes the steps of: providing a decontamination system including a first and a second tube, means for producing a decontaminating liquid having a ozonizer and an organic acid supply device, and a decontaminating liquid circulating pump connected to the first and the second tube; inserting the first and second tube into the pipe; and supplying the decontaminating liquid into the pipe through the first tube and discharging the decontaminating liquid through the second pipe so as to circulate the decontaminating liquid through an interior of the pipe and of the coolant circulating pump, while a level of the decontaminating liquid in the pipe is maintained so that the interior of the coolant circulating pump is filled up with the decontaminating liquid.